This invention relates generally to the manufacture of components used in complex machines such as aircraft engines, and more specifically to the process control of burnishing operations in production.
As is well known and widely described in the turbine engine art, components such as the blades and vanes in such complex machines operate at high mechanical and thermal cyclic loading conditions. In addition they are often subject to high cycle vibratory stresses during operation. Cracks initiating from low cycle and high cycle fatigue loading conditions, or from foreign object damage (FOD), may propagate in the presence of excessive tensile stress loading conditions. One way to improve the fatigue life of components is to induce compressive stress conditions at selected locations within the component.
There are known methods to induce compressive residual stresses within components during manufacturing. The use of shot peening during manufacturing is well known in the art. In shot peening, a stream of media is directed at the surface of the component at high velocity, causing plastic deformation and residual compressive stresses in the component. Laser Shock Peening (LSP) is another method that has been used successfully to induce residual compressive stresses in components. Laser shock peening typically uses multiple radiation pulses from a laser to produce shock waves on the surface of a component which induces residual compressive stresses. Another method of inducing compressive residual stresses within components is by precision deep peening with a mechanical indenter, for example as described in U.S. Pat. No. 5,771,729 issued to Bailey et al. and assigned to the assignee of the present invention.
Burnishing methods such as Deep Roller Burnishing (DRB) and Low Plasticity Burnishing (LPB) have been used in manufacturing for various purposes, including the inducement of residual compressive stresses in components. In such processes, a burnishing element such as a roller or ball is pressed against the surface of a component and moved along a selected path on the component. The pressing force used during burnishing is such that it induces plastic strain and residual compressive stresses within the component near the burnished region. Burnishing tools are typically hydraulically operated, using a pressurized fluid to force the burnishing element onto the surface of the component. Mechanically loaded tools are also used.
Although conceptually simple, burnishing processes need methods to control their results in a high volume production environment. There are several parameters, such as fluid pressure, volume flow, spring loads, surface conditions, lubrication efficiency, burnishing element wear, etc. that can influence the residual stresses obtained from burnishing. Currently burnishing process control relies primarily on freezing all parameters and tooling, and inferring that the end result of the burnishing process is adequately controlled. Although some of the machine control parameters such as pressures, speeds etc. can be can be monitored during manufacturing, these generally are not adequate to verify process control variations from other sources. Geometric measurements and visual assessments provide only limited evaluation of the burnished component. The beneficial residual stresses imparted to the interior region of the burnished component cannot be easily measured non-destructively. Accordingly, there is a need for a device and method to enable burnishing process control that simulates the entire process as applied to a component in production without the need for frequent, expensive, or destructive evaluations of the treated components.